1. Field of the Invention
The present invention relates to a capillary-retaining structure for an ultrasonic horn of a wire bonding apparatus.
2. Prior Art
Some wire bonding apparatuses use ultrasonic horns. In these apparatuses, a capillary is attached to the ultrasonic horn, and the ultrasonic horn is moved vertically and horizontally so that a bonding wire passing through the capillary is connected between the electrodes of semiconductor pellets and the lead posts of external leads.
One example of the conventional capillary-retaining structure used in an ultrasonic horn is shown in FIG. 5.
The ultrasonic horn 10 is provided at its end portion with a capillary attachment hole 11 which holds a capillary 30. A slit 12 is formed so as to extend from the tip end to the inside of the ultrasonic horn 10. The slit 12 is oriented perpendicular to the axis of the capillary attachment hole 11. A screw hole 13 into which an attachment screw 31 is inserted is formed in the ultrasonic horn 10 so that it is on one side of the slit 12. In addition, a threaded hole 14 with which the attachment screw 31 is screw engaged is formed concentrically with the screw hole 13 so that the threaded hole 14 is located on the other side of the slit 12.
Accordingly, when the attachment screw 31 is tightened, the slit 12 is narrowed and the capillary 30 in the attachment hole 11 is retained by the ultrasonic horn 10. When the wire (not shown in the drawing) which passes through the capillary 30 is going to be bonded to the electrodes of semiconductor pellets and the lead posts of external leads, ultrasonic waves are applied to the ultrasonic horn 10 from an ultrasonic vibrator (not shown) so that the capillary 30 is vibrated.
However, the prior art described above has problems. When the attachment screw 31 is tightened, the resulting stress is concentrated around the threaded hole 14 of the ultrasonic horn 10. As a result, the area 15 of the threaded hole 14 that is located near the slit 12 is warped toward the capillary 30, and the retention of the capillary 30 becomes unstable. If the retention of the capillary 30 is unstable, the vibrational characteristics of the ultrasonic horn 10 become unstable, and the bondability deteriorates.